A number of industrial applications require a suitable vacuum level to be kept in a sealed space for a period of some years. For example, this is the case of cathode ray tubes, also known in the field as CRTs, which are used as television or computer screens. Vacuum is required in the CRTs in order to prevent the trajectory of the electrons emitted by the cathode from being deflected due to collision with gas particles. In order to prevent this, CRTs are evacuated in the manufacturing stage by means of mechanical pumps and then hermetically closed.
However, it is known that vacuum in the tube tends to decrease with time, above all due to degassing of the internal components of the tube itself. It is therefore necessary to use a getter material inside the tube, which is capable of binding the gas molecules and thus preserving the vacuum degree necessary for the desired functioning of the cathode ray tube. For these purposes, barium is commonly used. Recently, the applicant has also proposed the use of calcium, which when compared to barium has the double advantage of being less toxic (thereby causing less problems in the manufacturing and disposing steps of cathode ray tubes) and of generating a reduced quantity of X-rays, injurious to health, when hit from the electron beam.
Due to the high reactivity of these metals, which would cause all the manufacturing steps to be problematic, some air stable compounds thereof are used, which are introduced into the cathode ray tube before its evacuation. In the case of the barium, the stable compound is BaAl4; in the case of calcium, it is possible to use CaAl2 or a ternary alloy Ca—Ba—Al containing between 53% and 56.8% by weight of aluminum, between 36% and 41.7% by weight of calcium and between 1.5% and 11% by weight of barium. These compounds are generally used in mixture with nickel and, in the case of calcium compounds, optionally or alternately with titanium. The ternary alloys are the subject of the Applicant's U.S. patent application Ser. No. 10/282,715, entitled “Device and Method for Producing a Calcium-Rich Getter Thin Film” filed Oct. 29, 2002, which is hereby incorporated by reference for all purposes.
In order to introduce these mixtures into cathode ray tubes, use is normally made of devices known to those skilled in the art as evaporable getters, formed of an upperly open metal container and containing powders of the desired mixture. Evaporable getter devices containing barium are for example described in patents U.S. Pat. Nos. 4,323,818, 4,553,065, 4,642,516, 4,961,040 and 5,118,988, all of which are hereby incorporated by reference. Examples of evaporable getter devices containing a calcium compound which can be cited are those described in international patent application WO01/01436 and in U.S. application Ser. No. 10/282,715 filed Oct. 29, 2002, entitled “Device and Method for Producing a Calcium-Rich Getter Thin Film” in the name of the applicant discussed above.
Once the evaporable getter device has been introduced into the cathode ray tube, the latter is connected to a vacuum pump and brought to the desired final internal pressure, generally lower than 10−5 hectoPascal (hPa). Finally, the evacuated cathode ray tube is sealed and heated from the outside by radio-frequencies in order to cause metal evaporation from the barium or calcium compound; then, the evaporated metal condenses onto the internal walls of the evacuated tube, thus forming the film active in gas sorption.
However, it is known that metal deposition onto specific areas of the cathode ray tube internal surface can be detrimental for the working of the tube itself or even totally compromise it. In particular, the formation of metal deposits on the screen and on the phosphors must be reduced as much as possible. Also the area between the electron gun (at cathode potential) and the so-called “anode button” must remain free from metal deposits because, as it is known by those skilled in the art, the presence of ionizable particles between two points at different electric charge would cause a short circuit of the system.
In order to prevent such drawbacks, it is possible to use particular measures such as evaporable getter devices provided with very high lateral walls, suitably formed so as to convey the evaporated metal jet onto some areas of the internal surfaces of the cathode ray tube; a getter device of this kind is described in U.S. Pat. No. 4,323,818. However, this method is not completely satisfactory, since the effect of directing the metal vapors is limited.
Alternatively, it is possible to use getter devices comprising deflectors positioned above the powder mixture of the barium or calcium precursor compound. Getter devices of this kind are described, for example, in U.S. Pat. No. 3,719,433. This solution, however, results in an increase of the time and consequently of the costs necessary for manufacturing said devices.